The membrane mechanism by which light is transduced into electrical signals were investigated in rod type photoreceptors of the toad using ionic substitutions and pharmacological studies. The rod photoresponse results from two processes, a light modulated mechanism and a voltage and time dependent mechanism. Techniques for isolating each mechanism and the contribution of each to the photoresponse was studied. The dark potential and the photoresponse were recorded as a function of the concentration of external sodium, potassium and chloride ions as were the effects of 4 amino pyrodine, cesium, and ouabain. From these measurements, it is concluded that in the dark, the rod membrane is 10 X more permeable to potassium than to sodium and that at the peak of the photoresponse, the sodium permeability is reduced by at least a factor of 10. We estimate that the cytoplasmic sodium and potassium concentrations are equal. These ionic gradients are maintained by active Na-K pumps.